Initially I thought they'd be put off by treated wood, but eventually they started drilling holes anywhere, and at some point I probably two dozen future residents (plus potential for kids later) which weren't really welcome.

I have since fought them with WD40, which seems to lead to a quick death, plus sprayed with a Citronella oil and other oils mixed with vodka to keep them away (seems to work temporarily) and have patched all the holes with foil and caulking on top of it.

I have also installed carpenter bee traps around the house, which basically get them into a bottle. I wonder what I can put into that bottle to kill them off relatively quickly/humanely, w/o having a smell that would prevent them from entering the trap.

Conserving wild bees

Research suggests that bumble bee boxes have a very low success rate in actually attracting bees into them. We find that if you create an environment where first of all you can attract mice inside, such as a pile of stones, a drystone wall, paving slabs with intentionally made cavities underneath, this will increase the success rate.

Most bumble bee species need a dry space about the size a football, with a narrow entrance tunnel approximately 2cm in diameter and 20 cm long. Most species nest underground along the base of a linear feature such as a hedge or wall. Sites need to be sheltered and out of direct sunlight.